Recently, as electronic devices have been more and more downsized and reduced the weight, printed circuit boards, especially, a flexible laminate (referred to also as flexible printed circuit board (FPC) etc.) has been demanded. The flexible laminate has a structure having a circuit comprising a metallic foil formed on an electric insulating film.
In general, the flexible laminate is formed of various electric insulating materials, and has an electric flexible insulating film as a substrate, and is manufactured by a method of laminating by heating and sticking by pressure a metallic foil onto a surface of the substrate with various adhesive materials. As the electric insulating film, a polyimide film etc. is preferably used.
As methods for laminating the metallic foil, a pressing method and a method of continuously carrying out lamination under a heated condition (thermal lamination method) are used. A multistage press, a vacuum press, etc. are used in the pressing method, and a double-belt press machine or a heated roll-laminating machine etc. is used in thermal lamination method. From a viewpoint of productivity, a thermal lamination method can more preferably be used.
In the thermal lamination method described above, appropriate conditions are selected in conformance with adhesive material to be used. When thermosetting resins, such as epoxy resins, are used for adhesive materials, usually, heating temperature at the time of thermal lamination (thermal-press forming device temperature) is less than 200 degrees C. (refer to Japanese publication of patent application; Japanese Patent Laid-Open No. 9-199830 official report (published on July 31, Heisei 9 (1997)) and Japanese Patent Laid-Open No. 10-235784 official report (published on September 8, Heisei 10 (1998))). Moreover, use of materials of thermally fusible type as the adhesive material, such as thermoplastic polyimides, usually requires elevated temperature of not less than 200 degrees C. as the heating temperature in order to develop thermally fusible property.
Here, heating temperatures of less than 200 degrees C. may decrease thermal stress applied to materials to be laminated, that is, an insulating film, a metallic foil, and an adhesive material, and therefore does not permit the tendency of occurrence of visual defects such as wrinkling at the time of thermal lamination in the flexible laminate obtained. On the other hand, heating temperatures of not less than 200 degrees C. increases dimensional changes caused by thermal expansion and thermal contraction of materials to be laminated, and therefore occurs a problem of visual defects such as wrinkling easily formed in a laminate to be obtained (laminated layer including laminated structure of insulating film/adhesive material/metallic foil).
A technique is proposed wherein disposition of a protective material between a pressurized surface and a metallic foil improves the above-mentioned visual defects during thermal lamination in case of using thermoplastic polyimides as an adhesive material (refer to Japanese publication of patent application: Japanese Patent Laid-Open No. 2001-310344 official report (published November 6, Heisei 13 (2001))). In the technique, since thermal lamination of materials to be laminated is performed using the protective material disposed on the outside of metallic foil, the protective material suppresses motions in a planar direction caused in the thermoplastic polyimide after thermal lamination. And therefore limitation in the motion of the thermoplastic polyimide enables suppression of occurrence of visual defects such as wrinkling.
However, the technique of disposing the protective material still has a problem of inadequate effect in a point of effective control of dimensional change caused in the laminated layer obtained even if it enables effective control of occurrence of visual defects.
Specifically, thermal expansion and thermal contraction of the materials to be laminated cause not only the visual defects, but occurrence of residual stress in the laminated layer after cooling. This residual stress becomes actualized as a dimensional change in the case of formation of wiring and circuit that have been formed in predetermined patterns by etching of the metallic foil.
In order to realize downsizing and weight saving of electric devices, in recent years miniaturization of wiring formed in a substrate progresses, and therefore miniaturized and high-density packaging parts are packaged. Therefore, when a dimensional change becomes larger after forming of minute wiring to cause a certain amount of shift of disposed position of parts in a designing phase, the shift leads to a problem of disabling excellent bonding between the parts and a substrate.
Thus, thermal expansion and thermal contraction of materials to be laminated at the time of thermal lamination has a large effect also on dimensional change. On the other hand, although the technique of disposing the protective material enables limitation of motion of the thermoplastic polyimide to avoid visual defects, it was difficult for the technique to effectively avoid occurrence of residual stress after thermal lamination. As a result, unavoidable dimensional change will occur after etching in the flexible laminate obtained.